What do I do? Changing my first name legally but confused about paperwork?

Okay, so I’m changing my name for various personal reasons. Lets say my legal name is Sam right now and I’m changing it to John (Not really… just random names, lol.)
I’m in the tenth grade, I have taken state tests (Regents) but this year I’m taking an AP exam. I’m also taking the SAT next year. I’m not sure when the right time to change my name is.
I really did not want to change it in high school but it seems that changing it after high school (AKA after I took my SAT, AP exams) would cause problems.
I don’t know what to do about this. I want my college name to say “John ___” but when should I change it? After I take my test and graduate high school or now? Will that cause too much confusion?